Page 7 - TunnelTalk Annual Review 2016

Extreme challenge for Rondout bypass
A6.6m diameter Robbins TBM is selected to operate under extreme conditions to complete the Delaware Aqueduct Bypass Tunnel repair project in the New York City water supply network.
The order is placed by the Kiewit/ Shea Joint Venture following award of the US$706.6 million Rondout tunnel contract in 2015. The deep bypass will prevent leakage of up to 20 million gal/day from the 72-year-old existing tunnel as it runs through the fractured limestone geology under the Hudson River. The Delaware Aqueduct tunnel conveys more than half of the drinking water for New York City. Owner of the US$1 billion project is the New York Department of Environmental Protection (DEP), which also completed project design in-house.
Full scope of works includes TBM excavation of the segmentally lined 4,000m long bypass tunnel; installation of a 2,800m steel interliner through the limestone section to prevent future leaks; excavation by drill+blast of 46m-long tunnels to connect both the TBM launch and retrieval shafts to the existing tunnel; and grouting work during drain-down outage of the aqueduct to prevent leakage from a lesser affected section of the main tunnel some 29km north of the TBM drive.
Kiewit/Shea began drill+blast excavation of the TBM bell-out chamber at the base of the shaft following completion in March 2016 of the two deep shafts under a separate US$101.6 million contract awarded to Schiavone. The chamber is scheduled for completion in 2017 ahead of arrival of the TBM.
The single shield machine will launch from the 845ft deep x 36ft o.d. (30ft i.d.) shaft situated on the Newburgh side of the Hudson River. From here it will pass some 100ft below the river under a maximum anticipated hydrostatic pressure of up to 20 bar, through to the 675ft deep x 38ft o.d. (33ft i.d.) reception shaft on the Wappinger side of the Hudson (Fig 1). Following completion, the shafts will remain in place as permanent access structures.
Robbins told TunnelTalk that the TBM is designed to hold a potential water
pressure of up to 30 bar in the passive mode using a cutterhead drive sealing system and annular back grouting through the tailskin.
The machine is equipped with high thrust to get through challenging ground and is fitted with sophisticated drilling and pre-grouting equipment for detection. Water-powered, high-pressure down-the-hole hammers are capable of drilling accurate probe and grout holes of 60–100m ahead of the TBM, while blow- out preventers enable drilling at pressures up to 20 bar.
The TBM will be built in the Robbins facility in Solon, Ohio, with a factory acceptance test scheduled for the last quarter of 2016. The machine will then be shipped to the jobsite for an estimated start-up in Spring 2017.
Anticipated geology features competent rock of densely compacted schists and shales at either end of a middle section under the river that includes a folded and faulted limestone formation which proved problematic during drill+blast construction of the original tunnel in 1939– 44. It is through this highly porous limestone section that the majority of leakage has been occuring.
Adam Bosch for NYDEP told TunnelTalk: “The original tunnel has a shorter steel liner through the limestone and detailed camera and sonar surveys using automated underwater vehicles (AUVs) showed that the most leakage was coming from the end of that liner.”
The steel liner in the new bypass will be considerably longer at 2,800m to prevent future leakage problems.
Installing structural steel support in the original Rondout Tunnel, 1942
TunnelTalk reporting
“During original construction in the 1940s, tunnel workers dealt with huge inflows of water,” said Bosch. “Total inflows were recorded at 2–4 million gal/day, with the largest single section of inflow coming in at 1,500 gal/min. This was inflow from both groundwater and river influence. Managing water will be a major challenge for the new project.”
To handle these expected inflows a grout curtain will be installed ahead of the TBM, and the contractor will also be prepared to pump out water where necessary. Despite the highly pressurized conditions at the face, hyperbaric interventions are not expected. “There are expected to be adequate sections of competent rock in which the contractor can perform maintenance in free air,” said Bosch.
Once the TBM drive breaks through, scheduled for 2020-21, flow into the Delaware Aqueduct from the Rondout Reservoir will be shut off. The 85-mile long tunnel will then be drained using a pumping shaft on the east side of the Hudson River close to the new bypass tunnel shaft. The final drill+blast connections of approximately 46m will then be made and lined with sprayed concrete. The portion of the existing tunnel that is being bypassed will then be plugged and abandoned permanently ahead of watering up of the new infrastructure.
During a shutdown period of approximately six months New York City will continue to be supplied by the Croton and Catskill systems. The bypass project has been in planning since the leaks were first detected in 1991, but until recently it would have been impossible to shut off one of New York’s three main sources of fresh water. Activation of the Croton Water Filtration Plant that will supply an extra 290 million gal/day during the shutdown period, will enable the six month window during which one of the city’s three supply tunnels can be shut down and the new bypass connections made. n
References
• Tunnel repair for aging New York aqueduct – TunnelTalk, September 2011
From left: Drill+blast shaft excavation; Fig 1. Cross-section alignment of the Delaware Aqueduct Bypass
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